1. Field of the Invention
This invention relates generally to automatic acquisition of fingerprints and other relieved-surface images, for access control--and to systems whose access is controlled by such automatic fingerprint etc. acquisition. The invention relates more particularly to fiber-optic prism systems for such fingerprint acquisition, and to cooperating mechanical and electrical provisions for both enhancing the identity confirmation and deterring circumvention of the identity confirmation.
Systems to which access is controlled in accordance with the present invention include personal weapons, other apparatus, facilities, financial services and information services.
2. Prior Art
Several U.S. patents address the topic of automatic control of hand weapons to deter unauthorized use of such weapons. At least one such patent, U.S. Pat. No. 4,467,545 of Shaw, suggests applying automatic fingerprint analysis to determine whether a prospective user is authorized. Some patents, such as U.S. Pat. No. 4,003,152 to Barker, incidentally propose use of more-transitory phenomena such as voiceprints and brainwaves.
Shaw's presentation is conceptual. The hand-weapon environment imposes formidable constraints of both time consumption and certainty in identification--as well as difficult secondary considerations including size, weight, shock resistance, physical reliability, and cost. As far as is known, no practical apparatus has heretofore been made or even devised that is capable of satisfying these constraints (or even most of them) to accomplish the tasks which the Shaw patent outlines.
Shaw offers the only known prior-art representation of a fingerprint-controlled personal weapon. That representation indicates pictorially that prints are to be taken from one or more of the four fingers other than the thumb--and at the point where these fingers are curled around the handle of the weapon.
It suggests no provision for holding any of the fingers in any sort of reproducible position or orientation, or with any sort of reproducible degree of pressure, against the weapon handle. It also suggests no provision for helping the user to remember (or circumventing the user's failure to remember), during stressful occasions of preparing to operate the weapon, the importance of pressing the fingers with some repeatable amount of firmness and in some particular position and orientation against the sensing surface.
Without any such precautions, success in verifying identity of an authorized user seems very unlikely. These comments are not intended as derogation of the Shaw patent--but rather only to illustrate that much remains to be done, to put his invention into practical use.
Now apart from the hand-weapon context, hundreds of United States patents address the topic of automatic verification of identity through automated acquisition and analysis of body data such as fingerprints, palm prints, or "grasping pressure"--or subdermal information such as blood-vessel or bone structures, transmission characteristics for electrical signals passing along arms, iris prints and even "antibody profiles". Still other hundreds of patents are directed to voiceprint recognition (and its twin field, the extraction of intelligence from speech).
To select from these many proposals a suitable identity-discriminating technology for use in controlling a weapon, it is necessary to take account of the practicalities which come into play. A hand-weapon environment demands, in combination and under adverse field conditions, both (a) near-instantaneous accessibility to input data and (b) a reasonably high level of identification certainty.
It is not clear that any of the above-mentioned body-data types, other than fingerprints and voiceprints, inherently can meet this dual demand--even putting aside the "secondary considerations" enumerated above.
The same is true, though time consumption is not as crucial, in many other practical field environments such as use of cellular telephones, portable computers, public phones, phone credit systems, and vehicles. Still other field systems such as automatic tellers and facility-entry access devices would be amenable to enhancement by ultraminiaturized fingerprint acquisition and verification modules.
Apart from traditional ink methods, the fingerprint-acquisition technology which appears to be most highly elaborated to-date is optical. Nonoptical alternative technologies have been proposed in several patents and if economic may be usable with certain aspects of the present invention and within the scope of certain of the appended claims.
These patents include U.S. Pat. No. 4,353,056 to Tsikos of Siemens, for a direct capacitive fingerprint sensor; U.S. Pat. No. 4,394,773 to Ruell, also of Siemens, for a piezoelectric fingerprint sensor; U.S. Pat. No. 4,526,043 to Bole of AT&T Bell Labs for a capacitive system with fingerprint-force-modulated carrier; and U.S. Pat. No. 4,577,345 to Abramov for an integrated-circuit sensor array overlaid with a pressure-sensitive membrane. (Also U.S. Pat. No. 4,788,593 to Ovshinsky describes a thin-film photosensor array. Some such device may be usable in place of a more-conventional photodetector array in virtually all aspects of the present invention.)
Notwithstanding these inviting alternatives, more-conventional optical techniques--CCD arrays, video etc.--are appealing for having been most widely analyzed and explored heretofore. These range from relatively primitive and direct taking of images by pointing optical devices at fingers, through interposition of transparent media to simply stop the finger, to more-sophisticated systems that set out to collect at the outset information that is--at least in concept--binary (i. e., not gray-scale or multivalued), through application of the principle of frustrated total internal reflection (FTIR).
In the latter category are two main groups of systems. One of these is typified by U.S. Pat. No. 4,728,186 to Eguchi of Fujitsu. Eguchi's FTIR-analogue system uses flat lighting, and evidently does not rely on incident/detected angle relations, but inherently distinguishes fingerprint ridges from fingerprint grooves--based upon the inability of light from grooves to exit toward a detection direction that exceeds the critical angle.
More-conventional FTIR systems are described in, for example, U.S. Pat. Nos. 3,947,128 to Weinberger; 4,783,167 to Schiller; 5,233,404 to Lougheed; and 5,210,588 to Lee of Goldstar (Korea). In all these systems except Eguchi's, light is incident on a solid/air interface from within the solid medium at an angle to the interface normal (perpendicular) that is just slightly greater than a critical angle for total internal reflection for the given solid at an interface with air.
The critical angle is defined by arcsin (1/n), a special case of Snell's Law in which n represents the refractive index of the solid material. Provided that the incident angle exceeds this critical angle and air is on the other side of the boundary surface of the solid, in purest principle all transmission through the interface is prevented (transmission is said to be "frustrated"). The incident light is all reflected back into the solid--substantially none can pass through--hence the term "total internal reflection" or "frustrated total internal reflection".
As is well known, reflection at a smooth surface is said to be "specular": the angle of reflection (the angle toward which the incident light is reflected) equals the angle of incidence. Hence the light which is subject to total internal reflection is geometrically confined--to generally the same extent as the incident light--to leave the interface surface with a distinct directionality.
If some other medium instead of air, however, is juxtaposed against the surface, then the effective critical angle rises to arcsin (n'/n), where n' represents the refractive index of the other medium. Under these circumstances, total internal reflection occurs only if the angle of incidence exceeds this higher critical value. If not, then only some of the light is specularly reflected internally, and the remainder passes through.
Conventional FTIR fingerprint systems take advantage of these relationships by directing incident light to such an interface, from within a solid clear prism or plate, at an angle which is intermediate between the critical angles arcsin (1/n) for air and arcsin (n'/n) for typical biological materials such as skin or flesh, and water. If no finger is present, or if a finger is present but a light ray strikes the surface at a groove of the fingerprint, then the light under consideration is all internally reflected as before.
If instead a light ray strikes the surface at a ridge of a fingerprint, then part of this light passes through the surface and into the material of the finger. As a practical matter the penetration is typically shallow, and in the course of this shallow propagation the light is not reflected specularly by the finger but rather is scattered diffusely.
The directional distribution of the scattered light includes a significant fraction (perhaps as much as half of the scattered light) that is returned through the boundary surface into the solid clear prism or plate, but propagating into a wide directional range--a full hemisphere. Thus only a small fraction of this backscattered light coincidentally travels along the same specular-reflection direction that would be taken, according to total internal reflection, in the absence of the finger or at a fingerprint groove.
The amount of this backscattered light that can be collected at any particular direction varies with the solid angle over which collection is performed. A lens may be used for the collection. If for example an f/1 lens forms a half-size image of a fingerprint on a detector, then considering geometry alone the fraction of backscattered light collected should be less than two percent of a full hemisphere.
This would suggest that less than one percent of the total intensity of light striking a ridge area is backscattered into such a lens. (This theoretical low value is subject to important degradations, as will be seen shortly.)
In one operating mode, light directed toward such a collector at the specular internal-reflection position but in the backscattering case can be distinguished, based upon its lower intensity, from that directed toward the same collector and position in the total-internal-reflection case. This distinction provides one way of distinguishing ridges from grooves, respectively.
From what has been said above, it will be clear that ideally the intensity ratio is on the order of a hundred, and is slightly further enhanced by the fact that some light entering a ridge is absorbed. Such a ratio, for this particular way of distinguishing ridges from grooves, is high enough to fairly characterize the distinction as conceptually binary.
In another operating mode, another way to distinguish ridges from grooves is to exclude collection of any specularly reflected light. The amount of backscattered light that can be collected in directions other than the internal-reflection direction, although weak, is much greater than its background.
As suggested above, for a like collection angle the backscattered light is ideally about one-hundredth of the incident radiation; it may be greater for favorable backscattering angles. In such "other" directions, assuming an ideally smooth surface, in idealized principle no light at all will be collected in the total-internal-reflection case.
Hence a conceptually binary distinction can be obtained for viewing of the backscattered light too, at viewing angles away from the specular-reflection direction. It is accordingly well known that conceptually binary data can be collected from an FTIR system, using either the so-called:
"bright field" case, the first operating mode discussed above, in which air-filled fingerprint grooves provide (through specular total-internal reflection) a bright background or "field" on which to view the dark lines corresponding to ridges; or PA1 "dark field" case, the second operating mode discussed above, in which air-filled fingerprint grooves provide (through absence of backscattering) a dark field on which to view the lighter lines corresponding to ridges. PA1 groove areas of both systems--bright for bright-field systems and dark for dark-field systems; but PA1 ridge areas of dark-field only. PA1 at the proper angle for FTIR operation at the termination (or, in some forms of the invention, first the proper angle for reflection at the termination--after which the fiber does duct the light to the finger-contacting surface at the proper angle for FTIR operation); and PA1 preferably in an angular relationship with the fiber which is not favorable to direct entry of the rays into a ducting mode. PA1 reflected at that termination into and along its fiber toward the electrooptical means, if the relieved surface is out of contact with that termination; and PA1 instead in large part--or at least fractionally--scattered by the relieved surface out of the corresponding fiber, if the relieved surface is in contact with that fiber termination. PA1 n.sub.avg .ident.average of core and cladding refractive indices in that region of the prism means; PA1 D.ident.periodicity of the fiber structure in that same region; and PA1 x.sub.F .ident.illumination-path distance across the prism means in that region, PA1 reflected at that termination out of its fiber, if the relieved surface is out of contact with that termination; and PA1 fractionally scattered by the relieved surface into and along its fiber toward the electrooptical means, if the relieved surface is in contact with the fiber termination. PA1 x.sub.M .ident.illumination-path distance across the prism means to the prism midplane, in the same region. PA1 an optical data-input end for contact with the relieved surface at the fiber terminations, PA1 at least one side face for receiving light into the prism means, and PA1 a partially reflecting surface for redirecting light received through the side face to illuminate the data-input end.
In the two cases light is incident on the finger-contact surface from essentially the same direction; and the interaction between the light, the solid clear prism or plate, and the finger surface relief are essentially the same. What differs is primarily the detector placement and preferably, in a conventional system, the collection-cone angle (related to numerical aperture).
As to placement, in the bright-field case light is collected for the detector along the direction of specular total-internal reflection. In the dark-field case light is collected in some other direction--most commonly, but not necessarily, about the normal to the solid/air interface surface.
The phrase "conceptually binary" has been used in the foregoing to make allowance for departures from ideal FTIR performance which are encountered in actual practice. Neither the supposedly continuous contact of a fingerprint ridge against the glass surface nor the theoretically clean separation of a fingerprint groove from the glass is perfect.
Due to these imperfections, observed intensity--and therefore contrast and signal-to-noise ratio--can depart dramatically from predictions of the simple geometrical theory introduced above. These departures in turn lead to significant preferences as between dark- and bright-field operation.
First, major intensity changes occur when separation is less than the wavelength of the light. In such cases, variations of a tenth of a micron--a significant fraction of a wavelength--can produce major fluctuations in intensity.
Relatively speaking, at fingerprint grooves separations roughly as small as a wavelength are only occasional unless a relatively large amount of liquid, dirt or skin detritus is in the grooves. The situation is different with respect to ridges: some parts of the finger may be contoured so that some ridges unavoidably graze or press against the surface too lightly.
Second and much more importantly, on a microscopic level fingerprint ridges are not continuous. They have myriad tiny gaps, as well as close and imperfect contacts, associated with the fibrous nature of the skin and the presence of sweat glands or pores.
Such grazing and gaps create macroscopic and microscopic pockets of FTIR specular reflection where the simple theory predicts only scattering. Some incident light that should be scattered is instead specularly reflected--or, in a manner of speaking, diverted into specular reflection. The resulting bright spots along a ridge cause the average or apparent intensity of the ridge--when viewed from the bright-field collection position--to rise toward that of the bright groove field.
Resulting intensity in a ridge region, in bright-field operation, is typically far greater than the predicted one percent of the incident light: in adverse cases it can be as high as seventy-five percent. This spurious brightness seriously degrades bright-field contrast.
Contrast C is often defined in terms of maximum and minimum intensities I.sub.max and I.sub.min as the ratio EQU C=(I.sub.max -I.sub.min)/(I.sub.max +I.sub.min).
If I.sub.max is the groove intensity, taken as one hundred percent, and I.sub.min is the ridge intensity, which is said above to be as high as seventy percent, then the contrast C can fall as low as (100-75)/(100+75), which is one-seventh (0.14).
When viewed as backscatter from a dark-field position, the fibrous and pore-induced ridge gaps create a series of dark spots along the ridges--which on average darkens the ridges from their ideal medium level of brightness. If there is some background level (that is, if the dark field is not perfectly dark) this darkening lowers the effective contrast relative to the dark grooves.
Such a decrease in contrast makes the ridges harder to distinguish from the grooves in the dark-field case too. It is therefore very important to avoid conditions that give rise to a significant background level in the dark-field case--but at least in the dark-field case if such care is taken to avoid significant background the contrast will be good, whereas contrast in the bright-field case is poor intrinsically.
As a result of the various ridge effects, (1) the resulting higher intensity from a ridge, in bright-field work, is closer to the bright groove intensity than is (2) the lower intensity from a ridge, in dark-field work, to the dark groove intensity if reasonable care is exercised. In short, contrast is at least potentially better in the dark-field system.
Grooves too can be troublesome. Their theoretically totally reflected, bright light may also be modulated--that is, subject to partial transmission, and corresponding reduction of reflected intensity in the bright grooves of the bright-field case.
As compared with the previously described effective "diversion" of backscatter into specular reflection (by pores along ridges), this is the converse: "diversion" of potential specular rays into backscatter. This occurs only where the fingerprint grooves are unusually shallow or a relatively large amount of liquid, dirt or skin detritus is in the grooves.
For the dark-field case this same modulation can cause modulation of the degree of darkness in the groove regions. Backscattered intensity increases, but again only where grooves are dirty or very shallow.
All these phenomena, in which both the theoretically reflected and the theoretically backscattered light components are in actuality modulated, occur in both dark- and bright-field operation. Preferences emerge, however, for dark-field operation.
Most often, intensity variations great enough to be troublesome occur in the variably-dark ridge areas of the bright-field case--but not in the relatively darker, or more consistently dark, groove areas of the dark-field case. Consequently the intensity is adequately well controlled in:
Accordingly dark-field systems are preferred for their higher contrast--ease of distinguishing ridges from grooves.
In addition to this preference based upon consideration of contrast itself, the same preference arises from considering the ratio of signal to noise--an essential consideration for precise measurements. As will be seen, the signal-to-noise ratio SNR is in part controlled by the contrast, so the contrast has an indirect effect as well as the direct one already discussed.
A well-known limitation on measurement precision at low light levels is so-called "shot noise" N--a physically unavoidable detector-signal random variation that is inherently proportional to the square root of the signal level S, thus: N.about..sqroot.S. To distinguish a ridge from a groove electrooptical-ly, it is necessary to detect a difference signal EQU .DELTA.S=S.sub.max -S.sub.min.
The effective noise N on this signal, however, is not subtractive but combines as the square root of the sum of the squares of the two noise levels, N.about..sqroot.N.sub.max.sup.2 +N.sub.min.sup.2 .about..sqroot.S.sub.max +S.sub.min . Therefore the signal-to-noise ratio SNR=.DELTA.S/N is proportional to ##EQU1## and this can be written in terms of the contrast C, as previously defined, so: ##EQU2## For purposes of the present discussion, the signals S.sub.max and S.sub.min may be understood as accumulated over a period of time t in response to the corresponding optical energy flows or intensities E.sub.max and E.sub.min, so that ##EQU3## This shows that the signal-to-noise ratio SNR is proportional to the contrast. This expression also shows that the signal-to-noise ratio SNR can be improved by raising the light levels E or the exposure time t, or both--but only in proportion to the square root of the exposure time or sum of the light intensities.
In the dark-field case the contrast C is almost always unity, because E.sub.min, the dark-field intensity at the grooves, is very small. Therefore the signal-to-noise ratio SNR is nearly always optimum in the dark-field case.
In the bright-field case, however, the contrast can be as low as about 1/7, with a proportionately lower signal-to-noise ratio SNR. The only way to make up for this lower contrast is to increase the exposure (that is, the light levels E or time t, or their product) by about 7.sup.2 =forty-nine times.
Low bright-field signal-to-noise ratio thus aggravates the low bright-field contrast. (Other preferences for dark-field use will be seen later for FTIR systems of the present invention.)
Because of these practical factors, real FTIR systems require careful engineering to optimize performance; this has been the experience with prior FTIR-based fingerprint readers for, e. g., countertop or office use. Often additional optical provisions are incorporated to provide a darker dark field, or otherwise improve either contrast or signal-to-noise ratio.
In attempts to adapt such systems for microminiature applications such as mounting in a personal weapon, even more troublesome practical limitations intrude.
A primary obstacle to use of FTIR clear prisms or plates in very close quarters is the need for a focal element such as a lens to image the FTIR data onto a detector array or scanning detector. Unfortunately a lens also has undesirable properties.
One such property is the need for focal distances between the lens and, at one side, the FTIR data or "object"--and at the other side, the image to be placed on the detector array or scanning plane. Focal distances, at both sides of the lens, ordinarily total several times the focal length.
More specifically the distance at the object side of the lens is (1+M)f, where f is the focal length and M the multiple of magnification or reduction. The distance at the image side is (1+1/M)f, for a total of (2+M+1/M)f.
For short-focal-length systems such as those of interest, the focal length roughly equals the transverse-diagonal dimension of the object. For a representative fingerprint, that transverse dimension is perhaps 11/2 to 2 cm; thus a minimum optical-train length is about 7 cm.
Further the lens too may be (perhaps particularly for the dark-field case) comparable in diameter to the object transverse diagonal.
To constrain the size and thus particularly the cost of a CCD or like detector array, it is usually desirable to use the lens to reduce (that is, demagnify) the fingerprint image at the detector. In this case the optical train lengthens to very roughly six focal lengths, or over 10 cm--and without greatly lowering the lens diameter.
This property conflicts with the need for extreme compactness, in the context of a personal weapon. While an optical path can be folded to overcome gross length, even this is awkward in a small space--particularly for a high-numerical-aperture or a wide-angle system.
A second undesirable property of a lens system is susceptibility to depth of field and distortion, particularly severe if the lens and object plane are not reasonably parallel and conaxial. Just such geometry is typical in bright-field FTIR devices, in which collection is necessarily accomplished (as explained earlier) off-normal by more than the critical angle vs. air.
Such systems have different magnifications at top and bottom of the fingerprint image, leading to complications in later interpretation of the acquired image. More seriously, the top and bottom of the image are badly out of focus when the center of the image is focused, unless a rather small numerical aperture is used.
This last conflicts somewhat with the state of the art in design of miniature portable apparatus, which is ordinarily directed toward minimizing battery size and maximizing battery life by minimizing lamp power--which is to say, maximizing light-gathering power. Another cure for the depth-of-field problem, but in essence one that is costly enough to validate the seriousness of the problem, is proposed in U.S. Pat. No. 5,109,427 to Yang of Goldstar: here a hologram is used to eliminate image tilt and resulting aberration.
Two prior patents have proposed substitution of a fiber-optic prism for a clear prism as a dark-field FTIR fingerprint collection block in a fingerprint reader: U.S. Pat. No. 4,785,171 and 4,932,776 of Dowling et al., assigned to Fingerprint Technology, Inc. of Pomfret, Conn. (A fiber-optic prism is made from one or usually more fused bundles of optical fibers.) This substitution offers relief in controlling some or all of the lens-system problems discussed above.
It appears from discussions in both these patents that corresponding apparatus has been built and tested, but also that Dowling is confused about what the apparatus in its reflection-based embodiments is doing. Despite careful introductions leading to discussion of the FTIR phenomenon, both texts ignore backscatter and steadfastly declare that the brighter parts of the image are due to skew rays reflected from the fingerprint grooves (in other words, a sort of bright-field operation).
Fingerprint grooves and ridges do look very much alike, and a ridge bifurcation is accompanied by the end of a groove in such a way that it could be mistaken for a groove bifurcation accompanied by the end of a ridge. It is certainly no discredit to the inventor that he was able to pursue these inventions with incomplete understanding of their technical behavior; however, the teachings of the patents are to this extent impaired.
In the first of these patents, Dowling retains a lens--using only the capability of the fiber prism to transfer the initially tilted image to a prism face that is parallel and conaxial to the lens. The lens is spaced away from that prism face in the usual elongated geometry (discussed above), also leaving the system vulnerable to scattering by contamination.
In Dowling '171 this problem is actually made more severe than in most conventional FTIR lens systems, because Dowling injects light into his system at this same output face of his fiber-optic prism. (Light ducted to the finger end of the prism meets a 45.degree. face there, which--wherever exposed to air--specularly deflects the light laterally out of the fibers for dissipation in the fiber structure. Where contacted by finger ridges, some of the light is backscattered into the fibers for return to the entry-and-camera face.) In consequence any light scattered by dirt at that face can proceed directly into his detector lens.
The severity of this problem is minimized in the '171 patent but acknowledged in Dowling's '776 second patent, which teaches use of a fiber-optic taper, integral with the fiber prism, to match the print image to a relatively small CCD array. It also teaches--instead of the spaced-lens configuration with injection and detection at the same end of the fiber-optic element--attaching a CCD array directly to the end of the fiber taper remote from the finger, thus entirely eliminating the lens and associated optical gap.
The fiber core has refractive index 1.62 and the cladding 1.48, yielding against air a moderately high numerical aperture NA=0.66 and critical angle of about 38.degree.. This choice is conventional for obtaining good light-gathering power, although many skilled artisans in this field would prefer a considerably higher numerical aperture.
(For the majority of current applications involving fused-bundle faceplates or image conduits, glasses with numerical apertures of 1.0 and 0.66 are used. Fused-bundle materials are also available with a very few other numerical-aperture values such as 0.95, 0.85 and 0.35; however, 0.95 or 0.85 face-plate material is not always available, and 0.35 is typically run "infrequently due to lack of demand"--see for example "Fiber Optic Faceplate Data", Incom, Inc., Southbridge, Mass.).
Here Dowling sets out to apply the full capabilities of the tapered fiber prism to shorten the optical system, erect the image plane (supplying an image that is merely anamorphic but in uniform focus and free of major aberration), and eliminate or minimize effects of contamination and jarring. Unfortunately, however, Dowling's fiber prism is covered by a CCD at one end and a finger at the other, leaving no suitable entry point for illumination.
Dowling proposes three alternative tactics for confronting this problem. In a first, the light is applied to the opposite side of the finger to be read--which in Dowling's term is thus "transilluminated".
This option appears unsatisfactory by virtue of at least three major drawbacks: bones in the finger degrade the uniformity of illumination, FTIR binary character is impaired or forfeited, and the optical path is longer and more elaborate--with a lamp outside the main housing and past the subject's finger.
As to the third of these points: if the lamp is far from the prism, light and power are wasted; if the lamp is close, new problems arise. The subject must slide a finger into position under the lamp, likely leading to distortion of the finger and thus the print if the finger is pressed against the prism while sliding. Also the subject cannot clearly see either the prism or the finger, which may cause some anxiety in some environments as for instance an automatic-teller machine. Such a system is unsuited for access control of a personal weapon, as the lamp housing would impede rapid grasping of the weapon.
In a second of Dowling's alternatives, small lamps "might be implanted in the face of the image sensor". This option is put forth with suitable tentativeness, as there readily appears no way of either (a) protecting the sensor from fogging by the lamps, or (b) causing light that originates outside any fiber to enter into that fiber--in the sense of being ducted along it to reach the finger end of the prism. This second alternative, not illustrated, would seem to be inoperative.
Dowling's third alternative is to direct the light into the sides of the fiber prism--specifically from the narrower sides of the taper--propagating toward the finger-contacting surface to be illuminated. In particular his illumination is directed into portions of the taper where fiber diameter is changing rapidly with respect to longitudinal position (i. e., the part of the taper that is actually tapered). Dowling teaches that the light should be thus projected into the prism from all four opposed sides (forming in effect a square light source), and at "30.degree. to 45.degree. relative to its major longitudinal axis".
Analysis indicates that this Dowling system will at best work very poorly, and most likely not at all. In particular, the efficiency of light injection in this manner is extremely poor.
Some very small fraction of light rays may possibly be injected into the inflection point, the most strongly tapered region, of the taper section. Through Successive internal reflections in the transition zone (from that angled region to the larger straight segment of the taper), rays so injected may possibly enter the ducted mode of propagation along the fibers and so reach the finger surface.
This process is the reverse of light leakage from a taper--in passage toward the narrower segment--as angles of inclination increase during multiple reflection in the wedged region. Although a significant fraction of light can be lost in such a way, as will be understood the loss is to stray light that then travels in many different directions outside the fiber bundle. Therefore, to use the reverse procedure in the injection process, very high illumination from any given narrow directional cone would be required to effectively illuminate the target finger.
In order to accomplish even this, however, it would be necessary to use a taper that has no absorbing material outside the individual-fiber walls (usually designated "extramural absorbing" or "EMA" material). In that case much of the returning backscattered light from fingerprint ridges would correspondingly escape from its fibers at that same inflection region of the taper, and these escaping rays would diffuse into many adjoining fibers and badly fog the image.
Any effort to overcome this problem as by, for instance, using fiber of much higher than usual numerical aperture would be counterproductive in that the injection mechanism described above, marginal at best, would be foreclosed (although this fact may well not be generally known).
Dowling indicates the importance of opposed illumination from all four sides of the taper, and this emphasis possibly flows from empirical observation that such illumination is needed to achieve operation. It is known in prior-art optical systems of various kinds to illuminate from, e. g., two opposed sides of a device to obtain more uniform lighting and thereby enhance performance--but this is a different matter from actually requiring such opposed or bidirectional illumination to make a system work at all.
Again, Dowling's discussion seems to suggest that his apparatus has been made and operated. If so, then it must operate at the very bounds of usability--a power-hungry system as it is working with small tail-end fragments of the input light that almost accidentally make their way to the fingerprint contact; and a low signal-to-noise system due to diffusion of major fractions of the backscattered light along the return path.
Such a system may appear adequate in simple bench tests and might actually be adequate in facility- and vehicle-mounted environments that can supply plentiful lamp power--and can afford either degraded identification certainty or extra analysis time to overcome marginal signal-to-noise. It would not be adequate in the extremely demanding low-power, high-certainty, fast-moving context of a personal weapon.
Dowling proposes only this opposed injection into the actually-tapered portions of the taper, and next to its narrower segments--and only a unitary fiber/prism assembly with 0.66 numerical aperture. He does not suggest any variation of the injection geometry, unitary prism, or numerical-aperture value. In view of the inoperability or near inoperability of the geometry, considered together with Dowling's own inability to explain its operation, the teachings of Dowling '776 while perhaps useful are not readily extended or refined.
Another technique for injecting light into a fused fiber-bundle structure, particularly an image conduit, is called "separate-channel illumination". Separate fibers are brought out of the bundle laterally for this purpose, and a light field is projected into these separate fibers--and carried in separate channels or layers in the direction opposite from the resulting image.
Although this technique may be useful for illuminating objects too close for other types of illumination, it would be inconsistent with the geometries needed for FTIR operation; and also is relatively expensive.
A very great body of patent and other literature relates to the interpretation and particularly comparison of fingerprint data once acquired. These may be performed visually or by automatic processors, such as computers using now-familiar digital electronic microprocessor technology.
Comparisons in most known systems proceed by abstracting, from collected images, details called minutiae. Generally speaking these are ridge terminations and bifurcations, stable scars and other permanent features.
Resort to minutiae has the very important advantage that minutiae and certain of their characteristics are usually invariant with age, weight change, position, and the distortion of a newly collected input image relative to a preexisting master image for comparison. The particularly useful characteristics are in essence topological near-invariants such as the number of ridges--or the approximate angular interval about a pattern center--which separate specified minutiae.
Use of minutiae is also, however, subject to the very important handicap that massive amounts of data handling are needed to in fact successfully abstract these data from a raw fingerprint image. Data processing requirements, in the context of very small automatic equipment such as personal weapons, are accordingly very demanding.
Here the entire algorithmic process of prenormalizing the image information, finding minutiae, determining their selected near-invariant characteristics, and comparing those with the characteristics for a given preexisting master image must all be accomplished within some small fraction of a second. Certain segments of this process may be subject to division into parallel analytical tasks for simultaneous processing, but a circuit providing such parallel processors may unacceptably increase overall system cost.
Portions of the present invention are compatible with making identifications through analysis of minutiae. This will be more clear from later portions of this document, including certain of the appended claims.
Some prior-art documents disclose use of "fingerbeds" or other devices for constraining the finger from which a print is to be read. These devices operate, with greater or lesser degrees of certainty, to control the position of the finger in relation to a sensing surface.
In general, however, relatively little attention has been devoted to constraining the orientation and firmness of finger application to the sensor. This observation has been first introduced above in relation to the Shaw patent, but is to an extent applicable to automatic fingerprint-acquisition devices more generally.
Even when supposed topological invariants such as minutiae are to be determined, such failure to provide reproducible finger orientation--and probably to a lesser extent a reproducible degree of finger pressure--introduces considerable uncertainty into the actual practical process of acquiring prints and thereby into the resulting identification. Although adequate identifications may be obtained using such equipment and methods, generally speaking more time and greater processing complexity is required in overcoming inconsistent input information, for any given degree of certainty.
As to personal-weapon access generally, the art fails to cope with a thief who bypasses ("hot wires") or removes access-control devices, making a weapon work without them.
Further the prior art has failed to provide an optical fingerprint reader module amenable to microminiaturization for access control in highly demanding field applications, particularly including personal weapons--and also encompassing access to use of portable computers and phones.
Public phones, phone credit systems, vehicles, automatic tellers and facility-entry access devices, although not as critical as portable personal equipment in terms of size, time, power, identification certainty, etc. would also be meaningfully enhanced by provision of a microminiaturized reader.
As now seen, prior art has failed to provide solutions to important problems; and important aspects of the technology in the field of the invention are amenable to useful refinement.